Re: Greetings

[Matthias Felleisen <matthias@ccs.neu.edu>]

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  > From: Chris Uzdavinis <chris@atdesk.com>
  > Organization: Automated Trading Desk, Inc.
  > Date: 08 Oct 2001 12:05:39 -0400
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  > "Paul Steckler" <steck@ccs.neu.edu> writes:
  > 
  > > > If anyone could explain what I've done wrong to warrent this warning,
  > > > I'd be very thankful.
  > > 
  > > The problem is that `cur' might be an empty list, in which case
  > > `car' would fail; the inferred type of `cur' is (listof sym), which
  > > includes the empty list.  If your loop had a test for null?,
  > > MrSpidey might be able to do better.
  > 
  > 
  > Isn't that just a theoretical problem?  As far as I can tell
  > there is no possible way for an empty list to ever be bound to cur.
  > 
  > I was under the impression that Mr. Spidy would report on problems
  > that it saw possible paths that could actually happen. 
  > 

Nope. MrSpidey sets up naive set-inclusions for the program and propagates
them thru the program. If there is a possible problem, it will indicate
this with red. It is then up to the programmer to decide (and document)
whether this is a weakness in the theorem prover or in the program.

  > It's been pointed out that the problem is with my "length"
  > counter, which was an attempt at optimization to reduce the number of
  > recursions when it could calculate the fact that there isn't enough
  > "available" items to possibly create another answer.
  > 
  > (I wasn't using it as an index, but rather a way of knowing that if we
  > want 'n' items from a list of length m, that we don't keep searching
  > in list m if it doesn't have enough items in the first place.  I
  > didn't want to just stop on an empty list, but also on a non-empty
  > list that was too small.)

I understand that you used it as a length rather than an index. See my
slogan. Can you give an example where the list is too short? 

  > 
  > Thanks to everyone for their feedback.  I'm going to investigate some
  > of the suggested alternate implementations.
  > 
  > -- 
  > Chris
  >